There is a growing demand for devices that are able to generate microscopic-sized liquid droplets, and in many cases to print onto solid surfaces. As a biomedical example, microarray technology has been developed to detect and analyze proteins and/or nucleic acid material (e.g., DNA or RNA) within a sample. These devices utilize highly parallel hybridization assays using an array of testing sites with deposited samples on a chip or slide. This technology has been useful in gathering information for genetic screening and expression analysis, as well as the detection of single nucleotide polymorphisms (SNPs). In addition, microarray technology can be utilized in other areas such as pharmacology research, infectious and genenomic disease detection, cancer diagnosis, and proteonomic research.
These microarray devices, however, require the formation of high-density hybridization sites or spots on a solid surface. The high-density array of test sites is generally formed using photolithographic patterning techniques, mechanical microspotting, or inkjet-like printing. The photolithographic method fabricates microarrays through on-chip chemical synthesis of DNA molecules using spatially directed exposure of light to selectively de-protect regions of the substrate. Affymetrix, Inc. of Santa Clara, Calif., for example, has developed this approach. While high-density test sites may be created using this method, there are significant manufacturing costs inherent in this method due to the use of light blocking masks and related lithographic equipment. In addition, lithographic processes, while suitable for large-scale production, is simply too expensive for small or intermediate scale productions.
In yet a second method, inkjet printing techniques are employed that forcibly eject fluid droplets from a printhead structure. The ejected droplets fly through the air and land on the substrate. While inkjet technology is mature and widely used in the case of traditional inkjet printers (used in the home and in business), the same technology cannot be directly translated into microarray applications. For example, in microarray applications, the droplets contain specific quantities of biological material (e.g., nucleic acids). Unfortunately, the number of samples deposited per area on the surface (i.e., average sample density on a spot) may vary widely because of splashing or spreading of droplets on the printing surface which could result in inconsistent hybridization data being generated.
In a third method, mechanical microspotting is used to print small amounts of solutions onto solid surfaces such as glass, silicon, or plastic substrates to form a testing array. The mechanical microspotting technique utilizes multiple fountain pen-like pins that leave droplets on the solid surface after contact is made between the pen “tip” and the surface. This method is generally simple and inexpensive for making a small number of microarray chips. Unfortunately, after repeated use, the tip of the pin (which is typically stainless steel) tends to deform plastically, thereby resulting in test sites having inconsistent spot size and shapes.
The pins used in microspotting have a capillary tract that contains the liquid. The liquid is dispensed from the capillary upon contact with the printing surface. The precision at which the liquid is retained in or released from the capillary is controlled by a number parameters including, for example, pin surface, print surface, printing speed, and ambient humidity conditions. During the formation of biological microarrays, great care is taken to control these parameters to ensure that the array of spots is formed in a precise and consistent manner.
In conventional pin-based microspotters, the pins are loaded with sample by dipping the tips of the pins into sample wells. Unfortunately, without a distinctive water-repelling property on the surface, excess liquid adheres or “clings” on the exterior surface of the pins. In order to remove this excess liquid, standard protocol requires that the pins undergo a pre-printing operation in which multiple large spots are printed on a dummy surface before printing on the intended printing surface. The pre-printing operation, however, wastes reagents, causes longer print times, and produces irregular sized spots.
Thus, there is a need for a printing pin that is overcomes the disadvantages of pins found in current microspotter devices. In particular, there is a need for a microspotter pin that reduces or eliminates entirely the adherence of residual fluid on the exterior of the printing pin. In this regard, there is a need for a pin design that does not require a pre-printing operation to remove adherent fluid.